Generally, as a brake disk for a motorcycle, there have been widely put into practice a brake disk including a mounting portion for mounting a wheel, and a sliding portion which comes into sliding contact with a brake pad, and formed by press-forming a sheet of metal plate; and a so-called floating brake disk including an annular sliding disk having a sliding portion which comes into pressing contact with a brake pad, a hub disk to be mounted on the inner side of the sliding disk, and a plurality of connection means for connecting the disks to each other in a floating state.
As the floating brake disk, there has been widely put into practice a configuration, in which a semi-circular cutaway portion is formed in a sliding disk and in a hub disk at an abutment portion therebetween, and a connection pin is mounted in a circular-shaped connection hole to be defined by abutting the cutaway portions of the sliding disk and the hub disk, whereby the two disks are connected to each other (see e.g. Patent Document 1).
On the other hand, as a connection structure of two disks in a floating brake disk, there is proposed a configuration, in which an inwardly projecting engaging convex portion is formed on the inner periphery of the sliding disk, the hub disk is formed with an engaging concave portion for receiving the engaging convex portion in such a manner that the engaging convex portion and the engaging concave portion are unmovable relative to each other circumferentially and radially, an engageable connection plate of a substantially U-shape in section is mounted on the outer periphery of the hub disk, and the connection plate is connected to the hub disk by a pin member in a state that the engaging convex portion is inserted in the connection plate and is engaged in the engaging concave portion, whereby the two disks are connected to each other (see e.g. Patent Document 2). Further, there is proposed a connection structure, in which fixing plates are alternately disposed on one surface and the other surface of a plurality of engaging convex portions disposed circumferentially at a certain interval, in place of the connection plate, and two disks are supported by the fixing plates to be axially unmovable relative to each other (see e.g. Patent Document 3).
Further, there is proposed and put into practice a brake disk, in which one or more groove portions are formed in a sliding surface of a sliding disk which comes into sliding contact with a brake pad in order to prevent noise at the time of braking, and to clean the sliding surface of the sliding disk which comes into sliding contact with the brake pad (see e.g. Patent Document 4).